1. Field of the Invention
The present invention relates to a method for fabricating a Bi thin film, and in particular to a Bi thin film fabricated by an electrodepositing method or a sputtering method and a spin electronic device using the same.
2. Description of the Related Art
Because of inherent physical characteristics and application possibility as spintronics device, researches on MR (magnetoresistance) effects of a thin film and a multilayer made of magnetic metal have been actively performed. In the MR effects, there is an AMR (anisotropic MR) effect shown in ferromagnetic metal and metal alloy. The AMR effect depending on an angle between a magnetization (M) direction of the ferromagnetic metal and a current flow direction in the metal has a comparatively low MR ratio less than 2%. In spite of the low MR ratio, the AMR effect was used widely for a reproducing head of a hard disk, etc. and a low-field sensor several years ago. In particular, the reproducing head uses a GMR (giant magnetoresistance) phenomenon discovered in a laminated structure of ferromagnetic metal and paramagnetic metal developed in the year 1988.
The GMR phenomenon was discovered in a Fe/Cr multi thin film for the first time, because it was possible to fabricate a multi thin film having uniform nano thickness (not greater than 1 nm) according to development of UHV (ultrahigh vacuum) techniques, the GMR phenomenon could occur. Because the GMR effect has a MR ratio several tens percent greater than that of the AMR effect, researches on the GMR effect have been actively performed all over the world. However, in the multi layer structure in which resistance is varied according to relative magnetization direction of an adjacent magnetic layer, because of very strong magnetic coupling between magnetic layers, very strong applied magnetic field is required, and accordingly it is difficult to apply the GMR phenomenon.
After developing a spin-valve structure in which magnetization of each magnetic layer occurs independently by inserting a non-magnetic metal layer such as Cu, etc. between two ferromagnetic metal layers, the GMR phenomenon can be applied to a device actually. In case of the spin-valve structure, because an inserted non-magnetic metal layer can have a sufficient thickness, magnetic coupling of adjacent two ferromagnetic metal layers can be eliminated, and accordingly it can act sensitively to very little external magnetic field of several 0e. Besides, there is an colossal MR, CMR effect occurred in manganite (La—Ca—Mn—O), etc., because it requires very great applied magnetic field and occurs only in a low temperature region, there are difficulties in applying it to a magnetic field sensor, etc.
There is a general MR (ordinary MR, OMR) effect occurred in all metal and metal alloy groups (herein, a MR (magnetoresistance) ratio of each metal is different). The general MR is regarded as the simplest among the MR effects, it occurs by variation of a moving path (according to Lorentz force affected by an external magnetic field) of a carrier in a substance. In case of general metal such as Cu and Au, etc., an OMR ratio is very little almost to be ignored. However, in some substances such as semimeatalic Bi, a MR effect is greater than a certain GMR effect due to electromagnetic characteristics of a substance.
Bi has a low carrier concentration and a very little effective mass as 1/100 of general metal. It has been reported bulk-shaped single crystal Bi has a very long mean free path not less than 100 μm. As mentioned-above, because of unique electric characteristics thereof, lots of researches have been performed on transport property of Bi, and it has been known Bi has good OMR characteristics. Because characteristics of Bi can be varied greatly according to a crystalline structure thereof, a crystalline structure of Bi is the most important factor in occurring of the OMR effect. With inherent characteristics of Bi, in applying of Bi to a device, it is essential to fabricate a Bi thin film of good quality not having a bulk shape and having good characteristics. However, it has been known it is difficult to fabricate a Bi thin film of good quality. Recently, fabricating a Bi thin film by performing MBE (molecular beam epitaxy) on a BaF2 substrate has been reported. In the MBE, a depositing speed is within a range of 0.1–1 nm/min, it is very slow in comparison with 10–100 nm/min in a sputtering method. Because the BaF2 substrate has a lattice parameter crystalline structure similar to that of Bi, epitaxy growing is possible on the BaF2 substrate, and accordingly it is selected as a substrate for growing a Bi thin film. However, the thin film is good only in the low temperature characteristics aspect, the thin film according to the conventional art is not practical in the film growing speed and production cost aspects, etc.